Electric furnace



May 2, 1933. T H` FORDE 1,907,290

ELECTRc FURNAGE Filed Deo. 2, 1929 2 Sheets-Sheet l JE?. .Z.

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IN V EN TOR.

4 TIL/05. H. FRE

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May 2, 1933. Tl H: FQRDE 1,907,290

4 ELECTRIC FURNAGE Filed Deo. 2, 1929 2 Sheets-Sheet 2 IN V EN TOR. THOS. H. FOD

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Patented May 2, 1933 UNITED STATES PATENT GFFICE THOMAS H. FORDE, OF SAN FRANCISCO, CLFORNA, ASSIG-NOR TO FORMELL CORPORA- TION, LTD., OF SAN FRANGISC, C vnFGRNA, A COBPGRATON OF CALIFORNIA ELECTRIC FURNACE Application filed December This invention relates to electric furnaces and in its present embodiment more particularly deals with 'the types of such furnaces that are used in the practise of dentistry, for the preparation of porcelain crowns or the like and in the elimination of wax from dental work.

i An object of the invention' to provide in furnaces of the character described a plurality of furnace chambers, commonly called inutiles, which are constructed, arranged, interrelated and operated in such manner that the various furnace operations required in the practise of dentistry, to prepare porcelain and other dental work, may be effected in less time, under greater capacity, at a lower cost and with greater ease and elliciency than heretofore.

Another object is to provide av furnace of the character described in which one electrically heated mulile is arranged to serve as the current control or rheostat means for itself as well as for another mufHe in such manner that the parts usually required for multi-muiie furnaces are materially reduced in number, the furnace construction is simpliliedy and reduced in cost, an economy of current consumption is effected, and a wide, varied and positive heat control is made possible.

` A further object is to provide for an etlicient porcelain furnace of the character described in which there may be embodied a large low heat muflle for preliminary drying purposes and a high heat mullle for finishing purposes, the said inutiles being electrically connected, constructed and arranged in such manner that the low heat muflle heating element serves 4as the rheostat for both muiies. Owing to the simultaneous operation -of such mailles, the capacity of the oven is materially increased, and a saving in time, effort and operating costs is effected. On the other hand, the particular electrical connection of said muflles including the utilization ofthe heating element of the low heat mufe as the resistance element of a rheostat Vwhich controls both muffles, provides a nicety of heat control, effects an economy in current consumption and, reduces the con- Serial No. 416,933.

struction and operation costs of the furnace, all in a particularly efhcient manner.

A still further object of my invention is to provide for oven construction of the character described in which a plurality, for cX- ample two, electrically heated inutiles may be arranged to be selectively employed the rheostat whereby either muffle while in operation may as desired be operated to control the other in a manner which particularly suits the furnace for use in eliminating wax from dental work but which may also be used for treatment of other dental work.

The invention possesses other objects and features of advantage, some of which, with the foregoing, will be set forth in the 'following description of the preferred form of the invention which is illust-rated in the drawings accompanying and forming part of the specification. It is to be understood, however, that variations in the showing made by the said drawings and description may be adopted within the scope of the invention as set forth in the claims.

Referring to the drawings,

Figure 1 is a vertical sectional view of a furnace constructed in accordance with my invention.

Figure 2 is a horizontal sectional view taken 0n the plane of line 2 2 of Figure 1.

Figure 3 is a diagrammatic view showing the electrical circuit for the furnace.

Figure 4 is a diagrammatic view showing the electrical circuit for a modified form of the furnace of the invention.

Figures 5 and 6 are perspective views of one of the muffles.

In carrying ont the present embodiment of my invention I employ two electrically heated inutiles in an arrangement whereby the heating element of one muiie serves as the resistance element of a rheostat for controlling the two inutiles, or l may arrange the element of each inutile to serve as the resistance element of a rheostat in such manner that either muilie may be operated to control the other. Furthermore l may provide one muffle to deliver a low heat and the other a high heat or may arrange both to give substantially the same heat, There high and low heat muffles are provided as a unit, it is preferable to arrange the low heat muffle to serve as the control or rheostat means and to provide a platinum or other highly resistant heating element for the high heat lnuffle and but an ordinary heating element say of nichrome for the low heat muflle. Such a furnace is particularly well adapted for treating porcelain crowns and other similar dental work. The low heat niuflle serves as a preliminary drying chamber and the high heat muflie as a finishing chamber.

Where the two mufiles are ea ch arranged to serve as a rheostat the heating elements are preferably of the same capacity and provide for a twin furnace which is particularly well suited for eliminating wax from inlays and other dental Work. Vith both types of furnace and in fact in any type wherein one electrically heated Inutile serves as the control or rheostat means for another muffle many advantages are provided compared to an ordinary rheostat controlled furnace. Not only are the parts reduced in number and the construction simplified and reduced as to cost owing to this novel feature of the invention, but the control of the heat is permitted with exceptional nicety, the capacity of the furnace is materially increased, an appreciable economy in fuel consumption is provided for, and the general efficiency of the furnace is manifestly increased. Nicety of heat control provides accuracy in creating and maintaining desired mufile temperature and owing to the presence of that highly desirable characteristic in the furnaces which. may be made in accordance with this invention, the proper preparation of porcelain and other dental work may be effected in greater capacity and in much shorter time than has heretofore been possible. The nicety and accuracy of heat control, the saving in time and the economy in current consumption provided for in accordance with this invention are of major importance in the practice of dentistry and mark a noteworthy advance in the art.

The specific embodiment of the furnace of this invention as illustrated in the accompanying drawings comprises a suitable casing or cabinet 1, here shown of rectangular form and provided with thermally insulated walls defining compartments 2 and 3 arranged to removably receive replaceable electrically heated muffles 4 and 5. These mufiles may be arranged to open on the top, botto-m.l or sides. The niuflle 4 is comparatively large and in the present instance is arranged so that access is had thereto through a door 6 in the front of the furnace cabinet. Access to the smaller mufile 5 is had through the open bottom thereof which is normally closed by a vertically movable platform 7 operated by a suitable means 8 including a hand Wheel 9 at the front of the cabinet, )l door 10 below the plane of the muflle 4 permits of access to the platform. By this arrangement the operator does not have to look into the muflle 5 and the objectionable eye strain usually caused by looking into the white heat of the muflle interior is done away with.

Preferably the muflie 4 is arranged to produce a low heat and is used for preliminary drying and kindred purposes in treating dental work of various kinds, especially the drying of porcelain crowns, or jacket or the like. On the other hand the muflie 5 is arranged to produce a high heat suitable for finishing operations on porcelain work and the like. Accordingly, a heating element 11, or nichrome wire or the like, is embedded in Walls of mufile 4 to produce a lower heat than the highly resistant element 12 of platinum or the like similarly arranged in the walls of the mufile 5. The differential in the heating capacities of these elements may be effected by varying the relative sizes thereof or by using elements of material having different coeflicients of resistance, or by both provisions.

In accordance with this invention I arrange one muflle to serve as the control or rheostat means for the other mufile. As here shown I tap coils of the resistance element 11 for the muille 4 as indicated by the numeral 14 and through conductors 15 leading from said tap points provide an arcuate series of contact members 16 and thus make a rheostat resistance unit out of said heating element. The movable or control element 17 of the rheostat is connected by a conductor 18 with a source of electrical current not shown. From the other side of said source a conductor 19 extends to and is connected with one end of the element 12 of the mufile 5 and from the other end of said element a conductor 20 leads to the element 12. Thus it will be seen that the two elements, including the rheostat, are connected in series and that the element 11 serves as a rheostat controlling the circuit and both elements. It will now be seen that the resistance energy of the low heat mufiie element 12, or in other words the heat generated by the rheostat-heating element unit of said muflle is not wasted but em loyed to heat the mufile 4 while a control oi) the energy to the element 12 of the high heat muflle 5 is be ing effected. When the rheostat is adjusted to reduce to the supply of electrical ener@r for the high heat mufile to the minimum, or to idle the high heat mufile, the maximum resistance is provided in the low heat muffle element 11 and said mufile will give off its maximum heat. Thus it will be seen that but little current is consumed when operating both mufiies. As the rheostat is operated to increase the current for the high heat muflie, the heat generation by the low lef-ai mufile is decreased inaslrlcuch as the suc- 4the element l1 in the circuit to the extent as desired, dependent on the amount of heat `to be given off in the flow 'heat furnace.

In order to insure a safe and lpositive circuitopening operation incident to the creation of an excessive temperature in the high -heat muliie 5, II arrange a thermo-fuse 2l interiorly ofthe mulle whereby the automatic voperation of said fuse will take place whether yor not the door or closure of the muflie is closed. Owing to the fact that the multle 4 opens at its lower side or bottom, the fuse is arranged to be suspended from the top of the muie, whereby the molten metal from vthe fuse will drop onto the platform or clotion.

It will now be clear that a precise and accurate heat control of the dual furnace, or better, of the two inutiles, is provided Yfor with an infinite nicety which will materially speedup the properpreparation of porcelain and-other-dentalwork while at the same time effecting an appreciable economy in such operations.

I may construct a furnace particularly for the purpose of eliminating wax from inlay matrices, inlays land other dental work, although suc'h operations may be carried out in'the furnace previously described. However,'to best suit the furnace for wax elimination work,the arrangement is as shown in the diagram in Figure 4, the main difference being linthe-electrical current and equipment rather than in the structural details of the furnace. yIn this special wax furnacelthetwo mufiies 23 and 24include heating element-s 25 and 26 which latter as here shown are of equal capacity. The coils of the 'heating elements and 26 are tapped and conductors 27 lead from the tap points-of the-elem-ent 25 to a row of spaced Contact points 28 whereas conductors 29 vlead from-the tap points of the element 26 to a similar and contiguous row of contacts'30. Thus the two elements comprise separate vresistance units of a two-way rheostat which has a lsingle movable contact member 3l having one end arranged to engage either set of the contacts 28 and 30. A conductor 32 electrically connects certain ends of the elements 25 and 26. A conductor 33 leading -froma source of electrical energy not shown is connected with the Contact member 3l. A similar line conductor 34 is connected with spaced Contact members 35 and 36 which are arranged `to be selectively engaged by a bridge contact member 37 carried `on the end of and insulated from themovable contact 31. The contact members 28 and 30 which are connected with the ends of the heating elements 25 and 26, are connected by conductors 38 and 39 with contact vmembers andel arranged to be engaged by the contact 37.

lith this arrangement it will be seen that when =the rheostat is adjusted as shown in Figure et so that one of thi` contacts 28 is engaged by the member 3l which-then disposes the contact 37 to engage the contactinembers and lli, the circuit then established so that the element 25 becomesthe rheostat for controlling the current to the element 26. Current then Hows from the conductor 33 through movable contact member 3l, the coupled part of the resistance element 25, conductor 32, element 26, the conductor 29 which leads from the other end of element 26 to the contact member 30 at one'end of the row of said contacts; then through conductor 39 connected with said contact 30, to the contact 4l; bridge contact 37, contact 35, and from thence back to the source through-conductor 34. Thus-it will be seen that-the heating element 25 of one mutlle serves as a vvariable resistance current control element forthe element 26 of the other inutile. `IVhen the resistance is high 4the heat in the mullle heated by element 25 is maximum, while a minimum vheat is provided in the muflie heated'by the element 26. As the resistance ofthemu'flle element-25m decreased theheat in the mutfle therefor is decreased, whereas the heatfproduced bythe mutlle element 26is increased. This arrangement may be reversed, if desired, and the heating element 26 made to serve as the rheostat. In this event the movable contact 31 is moved toengage the contacts 30 of the element26. -Current then flows through -conductor 33, contact 3l, one of the-contacts 30, through 'the then coupled portion cf the heating element 26, conductor 32, the full element 25, conductor 38, contact 4:0, bridge contact 37 then bridging contacts 40 and 36, and contacts 36 and conductor 3l`baclr to the source. By-this reversible arrangement, either muflie may be operated to produce a higher heat than the other, selectively as desired. This is advantageous in removing wax from dental work, as it does away with the necessity of transferring the work from one muflie to another when a change of temperatureVis-required for a given batch of work. Furthermore this selective arrangement permits a quick change from low to high heat and permits of a nicety and accuracy of heat control..

Although this arrangement is primarily intended for use in a wax eliminating furnace, it may also be used with elicacy in other ty es of furnaces. In this connection it is to e noted that the furnace developments of the invention are not limited for use in dental furnaces but are of general application in electric furnaces and will provide advantageous features owing to the nicety of heat control and the economy of construction and operation thus made possible.

An important feature of the furnace construction of this invention is the provision for ready and easy removal of the muiiles and for a quick replacement of a new mullle, in case a muile burns out or becomes otherwise inoperative or unfit for use. It will be noted that the cabinet or casing 1 is arranged to form suitable seats or sockets for said mules, which seats or sockets permit of said ready and easy withdrawal and replacement. As

shown in Figure 5, the mulles may be provided with contact members 45 arranged on the exterior thereof in such manner that they will engage similar casing carried contacts not shown, when the muliles are properly 1nserted into place in said casini. The use. of such contacts makes for a quie ly detachable electrical connection of the mullles in the operating circuits therefor and does away with the necessity of rewiring the furnace each time a muille replacement is made. Without these contacts each mulle would have to have the several conductors leading therefrom disconnected or removed and then re laced on the new or replaced mule.

It is important to note that the muilies of my invention are complete units including the heat control or rheostats which are built in and form parts of the muiiles. This complete unit arrangement is a decided advance in the art inasmuch as ordinarily rheostats suitable for use with the muflles of dental furnaces are not usually available on the market and must be especially made for each furnace and installed as separate units.

I claim:

1. In an electric furnace, a plurality of mules, an electric heating element for each muflle, spaced contact points electrically connected at spaced points of and with each heating element, a movable contact member disposable into position to engage the contact points of either element, and an electrical circuit arranged to permit operation of the movable contact to selectively electrically connect in the circuit either a part or the whole of either of said elements, with the other of said elements in series therewith.

2. In an electric furnace, a plurality of heating chambers, a resistance element arsistance element and connect a variable portion of such element in series with the other resistance element, the arrangement being such that either resistance element may serve to heat its chamber while it is functioning as a rheostat element for controlling the flou of current to the other resistance element.

4. In an electric furnace, a plurality of electrically heated mules, a power circuit therefor, a switch mechanism in said circuit operative to simultaneously and inversely vary the heats liberated in said mutlles whereby upon actuating said mechanism the heat of one muiiie may be selectively increased to a maximum heat while the heat in a second muflle is proportionately decreased to a minimum heat, and upon further actuation of said mechanism the heat of said first mullle may be selectively reduced from a maximum to a minimum heat while the second mule is simultaneously and proportionately increased from a minimum to a maximum heat. 5. In an electric furnace, a plurality of muilies, an electric heating element for each muiile, spaced contact points electrically connected at spaced points of one of said elements, a movable contact member disposable into position to selectively engage said contact points, and an electric circuit arranged upon movement of said member to simultaneously and inversely vary the heat liberated by said elements, whereby upon movement of said member in one direction the heat in one mullle may be increased to a maximum heat while the heat in a second mutlle is proportionately decreased to a minimum heat, and upon a reverse movement of said member the heat of said first muflle may be reduced to a minimum heat while the heat in said second mutile is simultaneously and proportionately increased to a maximum heat. 6. In an electric furnace, a plurality of muilles, an electric heating element for each mutlle, contact members electrically connected at spaced points of and with a pair of said elements, a pluralityV of electric circuits for connecting said elements to a source of electric power and terminating on additional contact members, one of said circuits connecting a fixed portion of one of said pair of elements in series with a variable portion of the other of said pair of elements, a second circuit connecting a fixed portion of said last element in series With a Variable portion of the irst element of said pair, and other Contact members arranged for selective engagement With the aforesaid Contact members to thereby select either of said circuits and to vary the portion of one of said elements in said circuit.

In testimony whereof, I have hereunto set my hand at 'Oak1and, California, this 25th day of November 1929.

THOMAS H. FORDE. 

